What is this type of texture called?
I guess it is the specular map. Specular maps are the maps you use to define a surface's shininess. Here's the example of a specular map I've found on net:


Here's how it works: First example (the one pictured below) doesn't use specular map. As you can see the gloss is evenly distributed on the model.



Second example shows the same model, but with the glossiness distribution based on the specular map.


To control on what parts you want the glossiness to appear you have to create a black and white texture and map it on your mesh (in my case pure black color is a 100% glossiness and white color is no gloss at all).


After setting the texture as a factor between Diffuse and Glossy shaders, you have your gloss appear on the desired parts of the model.


Here's the interesting tutorial about how to create the specular maps:
https://www.youtube.com/watch?v=5Gqp4bNJ52M